This invention relates to a signaling communication method in a mobile communications system. More particularly, the invention relates to a signaling communication method in a mobile communications system for encoding voice data to compress the amount of data, putting the compressed data into frame form, sending and receiving the frame data between a mobile station and a device on the side of a network, and performing communication by bypassing a vocoder of the network device when mobile stations communicate with each other.
In a mobile wireless communications system, as shown in FIG. 19, a number of base station controllers (BSC) are connected to a communications network NW and a plurality of base station transceiver subsystems (BTS) are connected to each base station controller BSC. Each base station is capable of two-way wireless communication with a mobile station (MS) within a radio zone (cell) centered on the base station. It is so arranged that voice data, which has been compressed by a variable-rate voice codec, and signaling data (data indicating a data handoff command and data indicating a set-up command when a call is originated and terminated, etc.) is multiplexed and transmitted in both directions between the mobile station and the base station controller BSC. To accomplish this, each of mobile stations MS-A, MS-B and each of base station controllers BSC-A, BSC-B are provided with a vocoder (VCD) for encoding voice data at a variable compression rate and restoring encoded compressed data to voice data. Further, each of the base station controllers BSC-A, BSC-B is provided with a call processor CLP and is adapted so as to implement call processing control and handoff control.
In a case where the mobile station MS-A and a stationary station (not shown) connected to the network communicate with each other, the vocoder VCD of the mobile station MS-A encodes voice data to compress the amount of data, places the compressed data in frame form and sends this data to the base station controller BSC-A via a base station transceiver subsystem BTS-A. The vocoder VCD of the base station controller BSC-A restores the encoded compressed data, which has been sent from the mobile station, to 8-bit PCM voice data and sends this data to the communication network NW, whence the data is sent to the stationary station. Further, when PCM voice data from a stationary station enters the base station controller BSC-A from the communication network NW, the vocoder VCD of the base station controller BSC-A encodes the PCM voice data to compress the data, places the compressed data in frame form and transmits this data to the mobile station MS-A via the base station transceiver subsystem BTS-A. The mobile station MS-A restores the input compressed data to the original voice data and outputs this data.
In a case where the mobile stations MS-A and MS-B communicate with each other, the vocoder VCD of the mobile station MS-A on the side of voice transmission encodes voice data to compresses the amount of data, places the compressed data in frame form and sends this data to the base station controller BSC-A via the base station transceiver subsystem BTS-A. The vocoder VCD of the base station controller BSC-A restores the encoded data, which has been sent from the mobile station, to 8-bit PCM voice data and sends this data to the communication network NW. The communication network sends the PCM voice data to the base station controller BSC-B on the receiving side. The vocoder VCD of the base station controller BSC-B encodes the PCM voice data to compress the data, places the compressed data in frame form and transmits this data to the mobile station MS-B via the base station transceiver subsystem BTS-B. The mobile station MS-B restores the input compressed data to the original voice data and outputs the voice data.
At the time of call processing control and at the time of handoff control, etc., it is necessary that the mobile stations MS-A, MS-B and the base station controllers BSC-A, BSC-B send and receive signaling data. The signaling data is transmitted upon being multiplexed into a fixed-length frame that carries the compressed data. In case of 8K CELP, one frame has a length of 172 bits and is transmitted at a period of 20 ms.
In a mobile wireless communications system, voice data and signaling data are transmitted between the mobile stations MS-A, MS-B and the base station controllers BSC-A, BSC-B upon being multiplexed into a fixed-length frame, as mentioned above. Consequently, when it is attempted to transfer signaling data from the base station controllers BSC-A, BSC-B to the mobile stations MS-A, MS-B at the time of handoff, etc., the base station controllers BSC-A, BSC-B lower the compression rate of the voice data from the full rate to less than the half rate, multiplex the signaling data into the space of the half-rate portion left empty by reduction in compression rate, and transfer the multiplexed data. In FIG. 20, (a) indicates an example of a frame in a case where voice data is transferred at the full rate. Here the frame does not contain signaling data. FIG. 20(b) shows an example of a frame obtained by reducing the compression rate to the half rate and multiplexing signaling data into the vacant space. The higher the compression rate, the higher the precision with which the voice data can be restored but the greater the number of encoded bits. On the other hand, the lower the compression rate, the lower the precision with which the voice data can be restored but the smaller the number of encoded bits.
When the mobile station MS-A and mobile station MSB converse with each other in a mobile communications system in which voice is transmitted upon being compressed using a codec in air intervals, the compressed voice data encoded and transmitted from the mobile station MS-A in the manner described above is decoded by the base station controller BSC-A and transmitted to a public switched telephone network (PSTN) as 64-Kbps PCM voice data. This PCM voice data is then encoded again by the base station controller BSC-B on the other end and is decoded again by the mobile station MS-B. As a result, codec processing is executed twice. A problem that arises is a decline in voice quality as compared with communication between a mobile station and a stationary station.
In order to prevent this decline in voice quality, consideration has been given to a system (vocoder bypass system) in which compressed voice data that has been encoded at a mobile station is transferred as is over a PCM line without being subjected to encode/decode processing by the base station controllers BSC-A, BSC-B. FIG. 21 is a diagram useful in describing a vocoder bypass system. When the mobile station MS-A and mobile station MS-B converse with each other, compressed voice data that has been encoded by the vocoder of the mobile station MS-A on the voice transmitting side is sent to the base station controller BSC-B on the other end via the communication network NW as is without being converted to PCM voice data by the vocoder of the base station controller BSC-A. The compressed voice data that has been received from the base station controller BSC-A via the communication network NW is sent to the base station controller BSC-B by being by-passed through the vocoder of the base station controller BSC-B. The same is true when voice is sent from the mobile station MS-B to the mobile station MS-A.
By thus bypassing the vocoders of the base station controllers BSC-A and BSC-B, compressed voice data encoded by the mobile stations MS-A and MS-B is transferred to the opposing mobile station as is and the data is decoded at the opposing mobile station. This means that encode/decoding processing need be executed only one time, resulting in much improved voice quality. In such a vocoder bypass system, however, a restraint is imposed upon the rate when voice is transmitted by the mobile stations MS-A, MS-B, and rate control within the vocoders of the base station controllers BSC-A, BSC-B cannot be carried out. As a consequence, if vocoder bypass is adopted in the conventional mobile communications system that controls the compression rate of the vocoders in the base station controllers BSC-A, BSC-B when signaling data is transmitted, rate control cannot be carried out and signaling data cannot be transmitted.